1. Field of the Invention
The present invention is directed generally to plasma processing chambers and, more particularly, to inductively coupled processing chambers.
2. Description of the Background
Plasma processing chambers are used in a number of different industries. For example, plasma processing chambers are used in the fabrication of integrated circuits, for coating medical devices, and for coating mirrors. Plasma processing chambers may be either inductively coupled or capacitively coupled. In the capacitively coupled systems, electrodes comprised of parallel plates are energized to produce the plasma. In the inductively coupled systems, an inductive coil is energized to produce the plasma. In both systems, varying the parameters of the mechanism used to generate the plasma provides some ability to control the characteristics of the generated plasma.
For example, in inductively coupled systems, the coil has a time varying radio frequency voltage impressed thereupon. The electron heating zone in the inductively coupled plasma is shaped like a torroid whose diameter is affected by the coil geometry. The value of the capacitor between the coil and ground may also effect the size and location of the electron heating zone.
In the semiconductor industry, a plasma chamber may be used to carry out a variety of processes such as etching, deposition, sputtering, and annealing. Many of those processes leave contaminant depositions throughout the processing chamber. Such contaminants may adversely impact the process step being performed which, in turn, can adversely impact device yield. The adverse impact on device yield becomes more pronounced as device size decreases.
Another problem is the wear out of the dielectric parts (the plate) within the processing chamber. Wear out of the plate is a particular problem when the chamber is used for etching. For example, an etching process can result in the deposition of a polymer on the plate. A cleaning step is required to remove that polymer. The efficacy of the cleaning step depends on a number of parameters, one of which is the value of the capacitive coupling between the coil and plasma. That capacitive coupling is defined by a voltage standing wave present on the inductive coil while powered. The higher the voltage, the greater the capacitive coupling. The cleaning efficacy at different radial and azimuthal locations on the parts varies with the value of the capacitive coupling. Also, the wear out of the plate is concentrated in one location determined by the value of the capacitive coupling.
Because of the need to keep expensive process equipment such as plasma chambers in service, it is desirable to operate the plasma chamber in a way which improves process capability, minimizes part wear, reduces time between cleans, and minimizes cleaning time. Thus, the need exists for a method and apparatus which achieves those goals.
The present invention solves the problems encountered in the prior art by controlling the voltage distribution of the standing wave impressed upon the coil, thereby controlling the location and amount of capacitive coupling. In the presently preferred embodiment, that is accomplished by controlling the value of the capacitor in the RF circuit between the coil and ground. Controlling the value of the coil to ground capacitor during an etching step controls the location of the deposit that occurs during an etching process. The capacitor can be fixed at an optimal value or varied from the beginning of the etch process (where the capacitor""s value may be less important) to the end (where the capacitor""s value can be very important). That permits the more critical end of the etch to occur in an optimally shaped electron heating zone and in a chamber that has an optimal deposition pattern of the polymer resulting from the capacitor value during the early portion of the step.
The invention also contemplates changing the value of the capacitor from the etch to the clean step and changing the value of the capacitor during the clean step. Changing the value of the capacitor during the clean step makes that step more efficient and eliminates the concentration of wear in one place. The value of the coil to ground capacitor may be changed stepwise or continuously during the etch step or the clean step.
More broadly, the present invention is directed to a method comprised of the steps of:
impressing a radio frequency voltage across a coil to establish a standing wave thereacross;
selecting a voltage profile for the standing wave so as to achieve optional capacitive coupling of at at least one predetermined position across the coil; and
controlling a circuit parameter, such as the value of a capacitor or inductor, to achieve the selected voltage profile.
An apparatus for carrying out the method is also disclosed. By controlling the voltage distribution of the standing wave, process capabilities are enhanced, the time between cleans is maximized, component wear is minimized, and cleaning time is minimized. Those advantages and benefits of the present invention, and others, will become apparent from the Description of the Preferred Embodiments hereinbelow.